Such a method is known from the printed publication “Charge comparison protection of transmission lines—relaying concepts” (Ernst, Hinman, Quan, Thorp; IEEE Transaction on Power Delivery, Vol. 7, No. 4, October 1992, pages 1834 to 1846). In this previously known method, the respective current is sampled at each end of a conductor, current samples being formed. The samples are picked up in this case at a time interval of 0.5 ms. The samples thereby produced are integrated individually at the conductor ends with the formation of measured charge values, the integration period corresponding to half a cycle length of the current—there being an alternating current with a fundamental frequency of 60 Hz in the previously known method. The integration intervals always start and end in this case with zero crossings of the alternating current measured at the respective conductor end. The measured charge values formed in this way are summed, specifically, on the one hand, with the formation of a scalar sum (“sum of absolute magnitudes”), by summing the measured charge values without taking account of the respective signs —that is without taking account of the flow of charge —and, on the other hand, with the formation of an arithmetic sum (“absolute magnitude of the sum of the signed magnitudes”)—termed total measured part value below—by virtue of the fact that the measured charged values are summed taking account of the respective sign. The error signal is generated whenever the total measured charge value (arithmetic sum) exceeds a threshold value dependent on the scalar sum, that is adapted to the respective measuring situation.
Furthermore, such a method is disclosed in U.S. Pat. No. 4,939,617. In this method, an error signal is generated upon the occurrence of an internal error on a power transmission line. For this purpose, measured current values are detected at both ends of the conductor, and these are integrated over a duration of half a cycle. The measured charge values detected in this way are transmitted via a communication line between the charge measuring devices in both directions, that is both from the first charge measuring instrument to the second charge measuring device, and vice versa. A tripping value (“restraint value”) is formed in evaluation devices assigned to the respective measuring instruments as the sum of the measured charge value of the first measuring instrument and the measured charge value of the second measuring instrument, by adding the measured values without taking account of their respective signs. Moreover, a reference value (“operate value”) is formed as a sum by adding the two measured charge values with the correct signs. This operate value is multiplied by a factor, for example 3, a weighted reference value being formed. An error signal specifying an internal fault on the power transmission line is generated whenever the operate value assumes a value that is greater than the restraint value.